The invention relates to a hydrostatic variable displacement unit having a swash plate and a servo system having a spring arrangement.
In the known hydrostatic variable displacement units having a swash plate, which operate as a closed-circuit pump or motor, the variable displacement pistons are guided in cylinders of a cylinder block and rotate about the shaft of the variable displacement unit. During the rotation, the displacement pistons are supported on the swash plate by means of sliding blocks, with each displacement piston executing a complete stroke during each 360° revolution. For this purpose, the swash plate has a planar running surface on which the sliding blocks.
The swash plate can be pivoted by means of a servo system in such a manner that the angular position of its running surface in relation to the stroke direction of the displacement pistons changes. The stroke of the said pistons therefore also changes as does the volumetric flow produced by the pump. The force required to change the pivoting angle of the swash plate is generally produced hydraulically by the servo system. For this purpose, the swash plate is connected to one or more servo pistons which are guided in corresponding servo cylinders and can be acted upon by pressure. The adjustment, brought about as a result, of the servo piston is transmitted mechanically to the swash plate which is thereby pivoted, for example via a servo arm which is connected to the swash plate. The spring forces of the spring arrangement for the resetting are dimensioned in such a manner that they return the pivoting angle of the swash plate into the neutral position, i.e. to the angular position of 0°, when the servo system of the variable displacement device is not activated.
Transversely situated servo pistons on closed-circuit pumps generally use servo springs which act in each direction of displacement of the servo piston because the resetting is thereby ensured for both pivoting directions of the swash plate using the same springs. In order to save on construction space, the springs may be accommodated in the hollow drilled servo pistons, but this gives rise to the problem that the servo arm of the swash plate is not able to apply to the servo piston a central force situated on the axis of movement of the servo piston and tilting forces unavoidably occur. If, on the other hand, the springs are placed on one side of the application of force into the servo space, these tilting forces are avoided, but a large amount of construction space is required. In order to reduce this construction width problem, the springs can furthermore also be placed into the servo-cylinder pressure space, but this requires parts which are manufactured very precisely, and is severely restricted in terms of the spring forces which can be selected because of the dimensions of the cylinder space.
One solution is disclosed in DE 100 37 482 C1. It describes a hydraulic variable displacement pump which operates in a closed circuit and has a servo system which is situated transversely and in which a double-action servo piston having two end surfaces guided in each case in a cylinder is provided. The piston is forced back into the neutral position by means of springs which act as compression springs during each piston stroke in any desired direction. In this case, the springs are arranged around the servo piston outside the servo-cylinder pressure space and, on both sides of the point of engagement of the servo arm, are supported on the housing in each case via spring plates and are supported on the opposite side in each case on one of the servo cylinders.
In the case of the known system, the servo arm can indeed apply force centrically, on the axis of movement of the servo piston. However, the fact that the springs are situated on a common central line about the axis of movement of the servo piston on both sides of the point of engagement of the servo arm has the drawback that the servo piston has to be in a number of parts in order to be able to fit the springs. In the case of the system according to DE 100 37 482 C1, the two parts forming the end surfaces of the servo piston are therefore screwed to the central part of the piston. In this case, during installation, the particular spring assembly between the spring plates has to be enclosed between the servo piston and servo cylinder in a manner free from play as far as possible and in the process positioned in such a manner that the pump is in the neutral position. The freedom from play makes it necessary to select the manufacturing tolerances of the individual components to be extremely exacting. On the other hand, the neutral position has to be adjusted by displacement of the entire servo system relative to the pump housing. A dedicated variable displacement housing is therefore required for the servo system. All of these circumstances make installation more difficult and increase the costs of manufacturing very considerably.
The primary object of the invention is therefore to provide a hydraulic variable displacement unit having a servo system which permits simpler manufacturing and installation.
This and other objects will be apparent to those skilled in the art.